An adjustable pulley is known from German Pat. No. 1 286 680. The support members which with their back areas define the circular-ring-shaped working diameter, have the design of symmetrical circular-ring segments. Each segment is divided into two equal halves by the radial plane which is defined by the associated radial guide groove. By rotating the disk, which has the spiral-shaped guide grooves, with respect to the disk which has the radial guide grooves, the segments are moved in the radial area which is defined by the disk diameters. This known pulley has proven to be excellent in practice. However, a limitation in its use results from the fact that, through the use of the symmetrical circular-ring segments as support members the pulley can not be adjusted for a very small working diameter. For this purpose the segments are extended too long in the radial direction in order to be guided sufficiently stable during adjusting movements between the disks. This disadvantage, that through this a small working diameter could not be achieved and only a low speed relationship between smallest and largest working diameter could be achieved, had to be accepted in view of a satisfactory functioning of the pulley during the adjustment.
A pulley which is variable in diameter is also known from German OS No. 20 30 334, the segments of which are symmetrical, namely their straight side flanks extend radially with respect to the axis of rotation. The segments slide on radial spokes, which is disadvantageous. A belt which is guided over the peripheries of the segments must run between said spokes, and hence contacts the spokes and thus damage to the belt and an untrue running cannot be prevented. Furthermore the pulley is wide and heavy. The spokes prevent a desired small diameter of the disk.
An adjustable pulley with only arced guide grooves in the disks is known from French Pat. No. 22 36 393, wherein the segments on one side are equipped with two pegs.
German Pat. No. 12 86 680 also discloses a pulley in which the segments on one side have a guide rib.
Knowing these disadvantages, the basic purpose of the invention is to improve an adjustable pulley of the above-mentioned type so that at a given diameter about the axis which carries the pulley, and at a technically tolerable outside diameter of the disk, an as large as possible adjustment area and thus an as large as possible relationship between the belt speeds at a smallest and largest adjusted diameters--on the assumption of a constant drive speed of the disk--can be achieved, and wherein during the adjustment a tilting of the segments and their jamming between the loosened disks is reliably prevented.
The aforesaid purpose is attained inventively in a pulley of the above-identified type by the measures which are defined in the attached claims.
The inventive solution is thereby based on the recognition that the force couple, which during adjustment is responsible for the tilting inclination of each segment, is directed in such a manner that it tends to tilt the segment at an inclined angle with respect to the respective radial plane, which is defined by the associated radial guide groove. This results from the guide grooves in both disks, which grooves intersect one another at almost 90.degree., and the pegs which engage the guide grooves and which are positioned on the two front sides of the segments in different positions. Through the asymmetrical design of each segment and the positioning of its longer diagonal in approximately the tilting direction, the segment is very securely supported against tilting. The support forces of the two disks, as a consequence of the force couple, act just in the direction of the longer diagonal providing the largest, optimum lever arms opposing the tilting moment. This measure results in an even, jerkfree movement of the segments during adjustment. This also permits, as a positive secondary effect, the radial extent of the segments to be substantially shorter than in the case of the state of the art. From this results the special advantage that, at a given axial diameter and constant speed, a substantially greater speed range exists. Same is, compared with known adjustable pulleys, approximately 30% greater. Stated differently, a known pulley would require at a required inner working diameter of 90 mm., for a speed control range of 1:3, an outside diameter of 270 mm.; while with the present invention under the same conditions an inner working diameter of 60 mm. and an outside diameter of 180 mm. can be realized.
The asymmetrical form of the segments results, in spite of the small radial extent of the segments, in advantageously long side surfaces. A further desirable effect is that, during adjustment of the segments from a smallest working diameter to a larger one, the segments totally separate from one another after only a slight relative rotation of the disks, and additional friction loads between the long side surfaces of the segments no longer occur. In spite of this, the segments can be manufactured simply and form, all together, a smaller mass than the segments according to the state of the prior art. Through this the moments of inertia of the pulley--in relationship to the various working diameters--become smaller than is the case in the state of the art. The segmentlike shape of the support members (i.e. the segments), of course, does not need to correspond with an exact circular-ring segment. In the same manner it is also possible to use support members which have the shape of a trapezoid with uneven long sides. The upper narrow side of the trapezoid can then be concavely rounded for contact with the splined tube. Also conceivable are asymmetrical segments, the sides of which are tailored or constricted in a top view. Support members of an I-shape with a convexly arced, short upper cross bar, which are connected by a radial web, can also be used. It is important only that a shorter and a longer diagonal exists, and the corner points (end points of the cross bars) lie for support on the disks at intervals which are caused by the geometric shape of the support member.
Even though the optimum design of the segments and of the angles of intersection of their longer diagonals with respect to the radial plane can most advantageously be determined through empiric tests, the angle of intersection of the long diagonal and the spacial position of the tilting force couple depends very strongly on the "lead angle" of the spiral-shaped guide groove. The respectively favorable angle of intersection is determined as explained hereinafter.